Neutralized transistor amplifier circuit



United States Patent 3,205,450 TRANSISTOR AMPLIFIER CIRCUIT NEUTRALIZEDThis invention relates to receiver systems and more particularly to ademodulating system for frequency modulation signals which is especiallyuseful as a sound signal amplifier and detector in a transistorizedtelevision receiver.

The present application is a division of application Serial No. 68,208,filed November 9, 1960, now US. Patent 3,122,708.

In many forms of frequency modulation detectors the circuit must bedesigned to operate over a very wide range of input signal levels. Forexample, in television receivers, the frequency modulation soundsubcarrier, generally at a frequency spaced 4.5 megacycles from the mainvideo carrier, can vary widely in amplitude as the receiver is finetuned. That is, the sound signal may fall anywhere from the maximumresponse of the intermediate frequency pass band to a region of greatattenuation Within the pass band. Overdrive of a transistor used as asound I.F. amplifier can lead to introduction of distortion in thedemodulated signal and undesirable amplitude moduation of the FM signal,or even oscillation of the system. Instability or oscillation may beparticularly bad with some transistors due to poor isolation of inputfrom output and an undesirable reverse gain characteristic.

It is accordingly an object of this invention to increase the range ofsignal levels which can be satisfactorily demodulated in a frequencymodulation detector system.

It is another object to reduce instability and distortion intransistorized frequency modulation amplifier and detector systems.

Another object is to provide an improved transistorized amplifiercircuit for a television receiver which is stabilized for high gain oflow level signals.

A feature is the provision of an amplifier, useful e.g., as a driverstage for a sound detector, having an inverse feedback network betweenthe collector and base circuits and a bridge balancing impedance for thenetwork to improve isolation between the input and output circuits andimprove stabilized high gain operation of the detector system.

In the drawing:

FIG. 1 is a diagram partly schematic and partly in block of atransistorized television receiver utilizing the frequency modulationdetecting system of the invention; and

FIG. 2 is a schematic diagram illustrating an equivalent circuit usefulin explaining the operation of the system.

In a specific form, the invention provides a transistorized sounddetector system for a television receiver. A video amplifier in thereceiver provides the sound subcarrier from the video detector and thissound subcarrier is coupled to a sound intermediate frequency amplifier,or driver, for a passive detector in the receiver. A portion of theoutput signal of the driver stage is applied from the collector of thedriver transistor through a capacitor to a further capacitor connectedin series with the base input signal source of the transistor to providestabilizing inverse feedback in the driver stage. The capacitors areselected to form a balanced bridge for neutralization of thecollector-base interelectrode capacity for stabilizing the system toimprove high gain operation, and a resistor-capacitor network is coupledto the neutralization balance point from the primary of the inputtransformer to minimize tendency for the transformer to unbalance thebridge. The ratio detector is effectively referenced to the emitterpotential of the driver transistor and a load resistor of the detectoris direct current connected to the base circuit of the driver transistorfor gain control' of the driver. The quiescent bias for the drivertransistor electrodes is adjusted so that under low level signalconditions the transistor is biased by the ratio detector for high gainand under high level signal conditions the driver is actually reversedbiased beyond cutoff and driven from cutoff to saturation by the signalto form an effective limiter circuit for driving the ratio detector.

Referring now to FIG. 1, the illustrated television receiver may be abattery operated type and all transistorized except for the picture tubeand high voltage rectifier.- The receiver includes a tuner comprising anRF amplifier stage 10, a mixer stage 12, and an oscillator 14. The tunerselects a particular television signal and converts the same to a signalof fixed intermediate frequency for further amplification in the IFamplifiers 16. Amplifiers 16 are connected to the video detector 20which demodulates a received composite video signal having line andframe synchronizing components, video frequency components, and amodulated sound subcarrier. The demodulated television signal is appliedto the first video amplifier 22 from which is derived the soundsubcarrier, generator frequency of 4.5 megacycles, and this signal isapplied to the sound amplifier 24. The sound signal is then coupled tothe sound detector 26 which demodulates the FM sound signal and appliesthe resulting audio frequency signal to the audio frequency amplifier30. Amplifier 30 is connected to a loudspeaker 32 for reproducing thesound.

The demodulated composite video signal is also applied from the firstvideo amplifier 22 to a second video amplifier 34 and from amplifier 34to the cathode ray picture tube 36. Tube 36 reproduces the videofrequency components of the signal to produce the resultant image of thetelevised scene. The synchronizing signal separator 38 is connected tovideo amplifier 34 and this circuit separates the line and framesynchronizing signal components from the composite video signal. Theframe or vertical synchronizing signals, generally at 60 cycles, arethen applied to the vertical sweep circuit 40 which develops a suitablesawtooth scanning current in a deflection yoke 42 which is disposed onthe neck of cathode ray tube 36. The separator circuit 38 is alsoconnected to the horizontal sweep and high voltage circuit 44 whichprovides line or horizontal signals, generally at 15.75 kc., in the formof sawtooth scanning currents in the deflection yoke 42. The circuit 44also provides a high voltage of the order of 15 kv. for the screen ofcathode ray tube 36.

The second video amplifier 34 supplies a signal proportional to thestrength of the detected video carrier to the AGC system 45, which isD.C. coupled through the video stages'to detector 20. The AGC system 45is connected to the RF amplifier and the IF amplifier 16 in order toregulate the gain of these stages in accordance with the strength of areceived signal in order to provide a uniform input signal level to thevideo detector 20.

It will be recognized by those skilled in the art that the precedinggeneral description of the television receiver of FIG. 1 refers to theoverall circuit functions in general terms. While in a specifictransistorized television receiver as being discussed, some of the abovementioned circuits will be non-standard it will be appreciated that theend result of the different circuits will be comparable to that of thecorresponding vacuum tube circuits, the operation of which is wellunderstood in the art. Accordingly, further detailed description ofthese circuits as presented in block form in FIG. 1 is believedunnecessary and the remaining description herein will relatespecifically to the construction and operation of the sound signaldetecting system of the receiver.

Reference will be made to frequency modulated signals to describe themodulated sound subcarrier of the composite television signal, but itwill be understood that this designation also may include phasemodulated signals.

Considering now the particular circuit connections in the system of FIG.1, the final stage of IF amplifier 16 is coupled through transformer 50to the video detector 20. One side of the secondary winding oftransformer 50 is bypassed to ground at signal frequencies throughcapacitor 51, the other side of the secondary winding is connected tothe cathode of a diode 53. The anode of diode 53 is connected throughthe choke 54 and the parallel combination of choke 56 and resistor 57 tothe base electrode of transistor 59. Base bias for the transistor 59 isformed at the junction of resistors 61 and 62 which are series connectedrespectively between ground and B+. The junction of these resistors iscoupled through the peaking coil 64 and the resistor 65 to theinterconnection of chokes 54 and 56. A further choke 67 is connectedbetween the junction of resistors 61, 62 and the signal groundedterminal of the secondary winding transformer 50. A capacitor 69 isconnected between the junction of diode 53 and choke 54 and ground. Afurther filter capacitor 70 is connected between the junction of chokes54, 56 andthe junction of capacitor 51 and choke 67.

Transistor 59 which is included in the first video amplifier 22 has anemitter electrode which is series connected with resistor 72 to B+. Theemitter is also connected through the series tuned circuit 74 to ground.Tuned circuit 74 is resonant at 4.5 megacycles in order to effectivelyground the emitter for the sound subcarrier so that transistor 59operates as a common emitter amplifier for this signal. It will also beseen that the emitter electrode is directly connected to the baseelectrode of a transistor in the second video amplifier 34. Since theresonant circuit 74 will have considerable impedance at,

the frequency of the video signal components, these are developed at theemitter electrode to be applied to the second video amplifier 34. Thecollector electrode of transistor 59 is connected in series to theprimary winding of coupling-transformer 76 and the'resistor 77 toground. The primary winding of transformer 76 is parallel tuned by meansof capacitor 78. This tuned circuit is used to develop the soundsubcarrier for application to the sound IF amplifier 24.

The sound subcarrier frequency 4.5 megacycles is developed across thesecondary winding of transformer 76 which is connected to the baseelectrode of transistor 80 and the sound amplifier 24. Another terminalof the transformer secondary winding is coupled through a capacitor 82to ground. Capacitor 84' is also coupled from this terminal of thesecond winding to the junction of the primary winding and resistor 77.Capacitor 84 has a low impedance at signal frequencies. A base biasresistor 86 is connected across the capacitor 82.

The emitter electrode of transistor is connected through resistor 88 to3+ in order to provide bias for this electrode and the emitter is alsobypassed to ground by means of capacitor 90. Output signals are derivedat the collector electrode of transistor 80 which is series connected toground through the primary winding of coupling transformer 92. Thisprimary winding is tuned by means of capacitor 94. The collectorelectrode of transistor 80 is also coupled through a small couplingcapacitor 96 to the junction of capacitor 82 and the secondary windingof transformer 76. The operation of the feedback network-includingcapacitors 82 and 96 will be explained subsequently. j

The sound subcarrier is developed across the secondary winding oftransformer 92 which is tuned by means of capacitor 98. One terminal ofthe transformer secondary Winding is connected to the anode of diode 100and the other terminal is connected to the cathodeof diode 101.Thecathode of diode 100 and the anode of diode 101. are interconnectedthrough the load resistor 103 and the terminals of this resistor arebypassed to ground for audio frequencies by means of capacitors 104 and105.

The tertiary winding of detector transformer 92 is inductively coupledvery closely with the primary winding, and it is connected from a centertap of the secondary winding through the resistor 107 to one side of avolume control potentiometer 110. The junction of potentiometer 110 andresistor 107 is bypassed for radio frequencies through capacitor 112 tothe junction of resistor 103 and capacitor 105. It will be recognizedthat the sound detector 26 is in the form of an unbalanced ratiodetector in which equal signal voltages of opposite polarity are appliedto the diodes 100, 101 and the signal induced in the tert ary winding ofthe transformer 92 is applied in parallel to these diodes. As thefrequency of the applied signal varies, the phase angle between the twosignal components applied to the diodes varies due to secondarytuning'and the instantaneous amplitudes of signals applied to the diodeschange at the modulation rate. The resulting difference in conduction ofthe two diodes provides a signal at the junction of resistor 107 andcapacitor 112 which is a function of the modulation of a PM. subcarriersignal.

A further fixed terminal of potentiometer 110 is grounded throughcapacitor 114 for audio frequencies and the variable arm of thepotentiometer 110 is connected through coupling capacitor 116 to thebase electrode of the audio frequency amplifier transistor 118. Itshould be understood that additional circuits will be included in theaudio frequency amplifier stage 30 in order to amplify the detectedaudio signal to sufiicient power to drive loudspeaker 32 and that thisamplifying circuit may be of known construction.

It will be recognized that in a television receiver as described thelevel of the sound signal may vary widely during use of the receiver.This can occur as a difficult problem due to adjustment of the finetuning of a television receiver. For example, generally the frequency ofoscillator 14 is varied by the user of the receiverto change slightlythe frequency position of the incoming signal with respect to the passband of the IF amplifier 16. By this operation it is possible to eitherposition the sound subcarrier at or near the maximum response of theamplifier 16 or at various other positions along one side of thedecreasing response of these amplifiers. In a prac tical situation finetuning may result in a variation of 50 db in the level of the soundsignal applied to the detector 26.

Ideally the signal applied to driver transistor 80 should be maintainedbelow an amplitude which would cause overdrive and instability thereof.However, in a practical situation the signal may vary from a level atwhich some amplification is necessary for proper operation of the audiosystem and throughout a range extending to high signal levels at whichthe transistor 80 would be overdriven thereby causing instability in theamplifier stage 24. The most effective operation of the transistor 80would be at a signal level wherein the transistor would be operated fromcutoff and driven into conduction by the input signal at its base. Underthese conditions the input signal level which could successfully behandled without reversal of the collector base diode would be maximized.However, this type of operation would minimize any signal gain for lowlevel signals since the transistor would be operated very close tocutoff. Thus, a dynamic biasing arrangement should be utilized in orderto forward bias transistor 80 for weak signals and reverse bias thetransistor for strong signals. This is accomplished in the presentcircuit by effectively floating the ratio type sound detector 26 at theemitter potential of the driver transistor 80 and deriving a dynamicbase electrode bias for transistor 80 from the positive potentialavailable from the detector load resistor.

To obtain the above described operation, a resistor 125 is connectedbetween the anode of diode 101 and the emitter of transistor 80. Aresistor 127 is also connected between the cathode of diode 100 and thejunction of resistor 86 with the secondary winding of transformer 76.Accordingly, resistor 127 is direct current connected through thesecondary winding to the base electrode of transistor 80. It will beobserved that since the detector 26 is not otherwise direct currentconnected to ground, it will be referenced to the emitter potential oftransistor 80. The positive direct current voltage appearing acrossresistor load 103, which voltage is proportional to the level of thesignal being detected, will be applied to the base electrode transistor80 through the resistor 127 as a bias control voltage therefor. Acircuit of practical construction can provide a small forward biasvoltage of approximately .2 volt with the base more negatve than theemitter for relatively weak signals. However, as the signal strength isincreased, this bias potential will be reversed and may rise to one voltwith the base positive with respect to the emitter.

In a dynamic biasing arrangement as described there may be some adverseaffect of the tuning of transformer 76 with such relatively wide biaschanges on the transistor 80. Any tendency for detuning can be offset bytuning the transformer 76 somewhat broader than normally, and while thismay adversely influence the matching of this tuned circuit to the outputimpedance of the first video amplifier transistor 59, this disadvantagecan be offset by the resultant increase of the input impedance of thetransistor 59 thereby improving the match of the second detector to theinput circuit of the transistor 59.

In the sound amplifier stage 24 a further cause of instability may bedue to the relatively poor attenuation characteristics of transistor 80in a reverse direction and therefore to improve the transistor operationwhen high gain is required, it is necessary to maximize the isolationbetween input and output circuits in the amplifier 24. Moreparticularly, oscillation or tendency for instability is overcome in thesystem of FIG. 1 by means of a degenerative or negative feedback circuitincorporated in the system in the form of a balanced bridge.

In FIG. 2 there is shown an equivalent circuit of a portion of the soundamplifier 24. This equivalent circuit is based on the so-calledhybrid-pi equivalent circuit of the transistor 80. FIG. 2 thusrepresents a common emitter amplifier incorporating negative feedback.As shown there is a resistance 140 in series with a capacitance 142between the base and collector electrodes of transistor 80. Furthermore,resistance 140 is in series with a further resistance 144 connected inparallel with a capacitance 146 between the base and emitter electrodes.The elements 140-146 represent internal resistances and capacitances ofthe transistor 80. Externally of the transistor there is capacitor 96connected between the collector and capacitor 82 and the effectiveinductance of transformer 76 is connected between the base electrode andthe junction of capacitors 96, 82. Capacitor 82 is connected to theemitter of transistor by way of the bypass capacitor which has very lowimpedance for signal frequencies. Resistor 77 is effectively connectedin parallel with capacitor 82 through the very low impedance of thecapacitor 84, and resistor 77 prevents the primary winding oftransformer 76 from reflecting a low impedance into the bridge tounbalance it.

The bridge circuit represented in FIG. 2 is brought into balance for theamplifier 24 to neutralize the feedback effect of capacitance 142 byproper election of the values of capacitors 96 and 98. It will be notedthat capacitor 96 forms a degenerative feedback path from the collectorof transistor 80 and the signal thus fed back will be developed acrossthe capacitor 82 (FIG. 1). The signal appearing across the capacitor 82and that developed across the secondary winding of transformer 76 areapplied between the base and emitter electrodes of transistor 80. Withthe bridge circuit as shown in FIG. 2 in balance, there will be a markeddecrease of the reverse gain of the amplifier 24 so that the reversegain of the stage can exceed the forward gain thereof in a negativesense by more than 14 db. Under these conditions it has been found thatsatisfactory isolation of the input and output circuits of thetransistor 80 can be obtained thus improving the utilization of the gainpossibilities of the transistor.

In a system of practical construction the parts values for the soundamplifier stage 24 were as follows:

Resistor 77 1,000 ohms.

Capacitor 78 22 mmf.

Transistor 80 High frequency Drift type or M.A.D.T.

type, e.g. T-1595 (Philco).

Capacitor 82 270 mmf. Capacitor 84 .01 mfd. Resistor 86 56,000 ohms.Resistor 88 68,000 ohms. Capacitor 90 .01 mf. Capacitor 96 8.2 mmf.Resistor 103 15,000 ohms. Resistor 15,000 ohms. Resistor 127 1,000 ohms.B+ potential 12 volts.

The foregoing invention provides therefore an improved frequencymodulation detection system particularly useful in a transistorizedtelevision receiver. The invention includes means for improving thedetection of signals over a wide signal input range thereby minimizingdistortion and tending to maintain proper receiver operation at both lowand high signal levels. Furthermore, the system includes provisions formaximizing the amplification of the sound detector driver and minimizingany tendency for instability in that stage.

I claim:

A signal amplifier including in combination, a transistor having base,emitter and collector electrodes, output circuit means coupled to saidcollector electrode for deriving signals from said amplifier, inputcircuit means for applying signals to said amplifier and including atransformer having primary and secondary windnigs, a first resistorconnected from said primary winding to a reference point, a firstcapacitor, means coupling said sec ondary winding in series with saidfirst capacitor respectively between said base electrode and thereference point, means connecting said emitter electrode to thereference point, a second capacitor having low impedance at signalfrequencies connected between the junction of said secondary windingwith said first capacitor and the junction of said primary winding withsaid first resistor so that said first capacitor and said first resistorare effectively connected in a shunt circuit between one side of saidsecondary winding and the reference point,'and a third capacitorconnected between said collector electrode and a the junction of saidsecondary winding with said first capacitor to provide negative feedbackin said amplifier, said first and third capacitors being selected toform a neutralization bridge together with the internal capacitances ofsaid transistor for reducing instability of said amplifier and wherebysaid second capacitor and said first resistor means minimize tendencyfor the impedance of said primary winding to unbalance saidneutralization bridge.

8 References Cited by the Examiner UNITED STATES PATENTS 2,769,869 11/56Mattingly 330-166 x 2,801,299 7/57 14110161 a1. 330-76 ROY LAKE, PrimaryExaminer. BENNETT G. MILLER, Examiner.

